1. Field of the Invention
The present invention relates to nanostructures and, more specifically, to a method for making elongated nanostructures.
2. Description of the Related Art
Zinc oxide (ZnO) is a piezoelectric material that has a direct wide band gap of 3.37 eV and a large excitation binding energy of 60 meV. It has been used in a variety of applications, including: electronic, optoelectronic, electrochemical, and electromechanical nanodevices, such as ultraviolet (UV) lasers, light-emitting diodes, field emission devices, solar cells, high performance nanosensors, piezoelectric nanogenerators, and nano-piezotronics. ZnO nanowires (NWs) have been synthesized by a wide range of approaches, such as physical vapor deposition, wet chemical method, pulsed laser deposition, metal-organic chemical vapor deposition, molecular beam epitaxy, etc. By controlling the synthesis parameters, a control has been demonstrated on their morphology, dimensionality, crystallinity, and aspect ratio of the ZnO NWs.
Typically, most ZnO nanowires have been formed as freely floating inside the solution body and entangled with each other. In order to integrate the ZnO NWs into functional nanodevices, a seeded growth method has been used to grow vertically aligned ZnO NW arrays by coating a substrate with a ZnO seed thin film prior to wet chemical growth. However, the ZnO seed film was polycrystalline in nature and the vertical alignment of the seeded grown ZnO NW arrays was rather poor. By employing an epitaxial relationship on monocrystalline substrates, almost perfectly vertically aligned ZnO NW arrays were grown on GaN, AlGaN, SiC, Al2O3, and MgAl2O4 substrates. In an effort to arrange the ZnO NWs into a more regular form to further enhance the performance of the nanodevices, the positions of the ZnO NWs on the substrate have been controlled by a variety of techniques, including photolithography, nano-sphere lithography, nano-imprint lithography and electron beam lithography.
Horizontally-grown ZnO NWs may have many potential applications. In growing such horizontal nanowires, instead of coating a top surface of the substrate, ZnO seed thin films are coated on the sidewalls of the trenches on the substrate so that the ZnO NWs are able to grow parallel to the substrate surface. However, the aligned ZnO NWs tend to be sparse and exhibit poor horizontal alignment. Horizontally aligned ZnO NW arrays can also be made on the top surface a substrate by controlling the surface texture of the as deposited polycrystalline ZnO seed film with pulsed laser deposition. Epitaxial growth of horizontal ZnO NW arrays on an a-plane sapphire substrate by physical vapor deposition has been attempted, but the method lacked uniformity and spatial control of the horizontal ZnO NWs. Horizontal alignment of ZnO NWs after growth has also been demonstrated by dispersing and floating the NWs in a solvent and then applying high frequency alternating electrical field. However, existing methods have had difficulty growing long horizontal nanowires.
Also, many potential applications of horizontally-grown nanowires require placement of the nanowires on a flexible substrate. However, existing methods of placement can be impractical.
Therefore, there is a need for a method for growing long horizontal nanowires and embedding them in a flexible substrate.